The present invention relates to a process for preparing clear and colorless alkoxysilane monomers from the reaction of a silazane with an organic alcohol in the presence of an inorganic acid catalyst. The inorganic acid catalyst is present in a critical range from 20 parts per million (ppm) to 300 ppm. Inorganic acid catalysts used within this critical range eliminate the need for a purification step in the process.
Silylation is the introduction of a silyl group into a molecule in substitution for active hydrogen. It is known that organic compounds can be silylated by reaction with certain types of organosilicon compounds. For example, U.S. Pat. No. 3,636,026 discloses a silylation reaction in which hexamethyldisilazane is reacted with benzoin and sulfuric acid at 125.degree. C., followed by a distillation to purify the crude product. It is important to note that the minimum amount of acid suggested in U.S. Pat. No. 3,636,026 is approximately 800 ppm. A silylation reaction is also disclosed in U.S. Pat. No. 3,529,006 in which N-methylolsilyl ethers are produced by reacting N-methylol compounds with an excess of hexamethyldisilazane in the presence of ammonium sulfate, a diluent is added and the product is filtered. In U.S. Pat. No. 3,529,006, the ammonium sulfate is formed in the silylation reaction using a minimum of 526 ppm of sulfuric acid.
The above mentioned processes have been employed successfully on a commercial scale for the preparation of pharmaceutical products. However, although the processes are capable of providing fairly good yields of the silylated product, the products formed by such processes must be purified by distillation and/or filtration before a clear and colorless product is obtained. The present inventors have discovered that haziness in the products of such processes is a result of the high levels of inorganic acids used in the silylation reactions. The level of inorganic acid employed determines the amount of the corresponding ammonium salt in the product. Such ammonium salts are insoluble and render the product hazy. A continuing need exists, therefore, for further improvements in silylation reactions to eliminate the purification process step while securing a transparent product.
The present inventors have discovered that the purification step in silylation reactions may be eliminated if between 20 ppm and 300 ppm of an inorganic acid catalyst is used. This was particularly unexpected since it was not known whether such small amounts of inorganic acid catalyst would take the reaction to completion. Moreover, inorganic acid catalysts employed within this critical range do not generate enough of the corresponding ammonium salts to render the product unclear. Thus, the alkoxysilane monomers formed from the process of the present invention are clear, colorless and odorless, and thus, may be formulated into a wide variety of products, such as cosmetics.